White Christmas Forecast at D-7: 30% Chance of “New” Snow

December 18, 2009 08:35 – 08:35

Or is it D-8? I guess it depends on how you count. “How” is an important question.

How exactly do you define “White Christmas”? Does that mean that the ground is white — regardless of how long ago it snowed? Or does it mean that it has to snow on Christmas day itself? If it’s the former, then a White Christmas looks very likely — a 90% chance at this point. If you mean the latter, given the highs and lows that currently are forecast, the chances for new snow accumulation on Christmas are not all that great, according to the Weather Channel.

Meanwhile, it’s looking like the weekend storm might really happen. It’s forecast to begin late tonight and end Sunday morning. The NWS is now forecasting 6-12″ by tomorrow night, with additional accumulation before stopping Sunday morning. So, my own forecast of 8-14″ is looking pretty good. Here are the NWS forecast icons for the weekend event in 22307:

They come with a Winter Storm Warning that goes from midnight tonight through 6 am Sunday for the DC area.

Looking ahead to December 25th, it’s still not definitive. I now think we’ll have our white Christmas, but the probability of new snow (from Xmas Eve or Xmas Day) is still pretty small. The Xmas Eve forecast is for an overnight low of 34°F, with a 60% chance of snow showers. Normally, at 34°F, snow showers wouldn’t do anything. But, it’s likely that we’ll still have at least 6-10″ of compacted snow on the ground. So anything new that falls — even at 34°, will not melt as it falls.

A complication for forecasting is that a major coastal storm like the one we’re probably going to get this weekend changes the dynamics — particularly if it’s snow. Snow cover will include most of Virginia, West Virginia, Tennessee, and parts of North Carolina. That means that any southern winds reaching us will be cooled by snow cover at the surface —reducing temperature forecasts. The current 43°F forecast for December 25th is based on current dynamics — which don’t include snow cover. Once we have snow on the ground, all post-event forecasts will need to be retooled. Temperatures aloft behave differently, but they aren’t unaffected. One way or the other, however, it’s looking like we’re going to beat the long-term 13% chance of a White Christmas this year.

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